Nextcloud version (eg, 20.0.5): 20.0.6
Operating system and version (eg, Ubuntu 20.04): Debian 10
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4
PHP version (eg, 7.4): 7.4.16
The issue you are facing:
Shared hosting (IONOS Web Pro).
Randomly, users are experiencing these 2 following errors:
unknown error has occurred
connection to servr lost
This happens on any app, not only Files, for instance Calendar.
No error message in nextcloud.log.
I could reproduce the error by uploading a 100MB mp4 file using FF 87.0.
I could trace the event and uploaded a HAR file here.
I did not find anything in this log.
The “event” happened at 2021-04-02T10:43:08.
The same upload has been working fine three times at different moments using Brave browser, making me think this might be related to the browser, in some way.
Is this the first time you’ve seen this error? (Y/N): N
The output of your Nextcloud log in Admin > Logging:
Nothing
The output of your Apache/nginx/system log in /var/log/____:
You should open a ticket with IONOS. Or perahps try a more performant hoster.
At Cloudeezy, you can try to reproduce the same error in their FREE version.
I use Firefox with my selfhosted Nextcloud Instance on a daily basis and have never experienced such problems. I really doubt that your problem has anything to do with the browser you are using. And even if it were, the root cause of the problem lies almost certainly somewhere else.